1. Field of the Invention
The present invention relates to an anode having an anode current collector and an anode active material layer providing thereon, a method of manufacturing it, a battery including an anode, and a method of manufacturing it.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery can provide a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery has an anode having a structure in which an anode active material layer containing an anode active material is provided on an anode current collector. As the anode active material, a carbon material has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, improving the battery capacity is further demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
However, when the anode active material layer is formed by depositing silicon as an anode active material with the use of vapor-phase deposition method, the binding characteristics are not sufficient. Thus, in this case, when charge and discharge are repeated, there is a possibility that the anode active material layer is largely expanded and shrunk to be pulverized. When the anode active material layer is pulverized, depending on the pulverization degree, an irreversible lithium oxide is excessively formed due to the increased surface area, and the current collectivity is lowered due to falling of the anode active material layer from the anode current collector. In the result, the cycle characteristics as important characteristics of secondary batteries are lowered.
Therefore, to improve the cycle characteristics even when silicon is used as an anode active material, various devices have been invented. Specifically, a technique in which the surface of an anode active material is covered with a metal such as iron, cobalt, nickel, zinc, and copper (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-036323), a technique in which a metal element such as copper not being alloyed with lithium is diffused in an anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-273892), a technique in which copper is dissolved in an anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-289177) and the like have been proposed. In addition, as a related art, a sputtering device including two sputtering sources arranged so that each plasma region is overlaid on each other for using two types of elements as an anode active material is known (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-007291).